The present invention relates to the field of hard disk drives for use in computer systems. In particular, the present invention relates to a technique for improving tracking of a write head and improving shock sensing in a hard disk drive system.
Areal densities for magnetic hard disk-drives continue to grow aggressively in recent years, with no sign of abating in the foreseeable future. As areal density grows beyond 10 Gb/in2 towards 100 Gb/in2, there is a need to shrink the bit aspect ratio from the present 15-20:1 down to about 4:1, in order to achieve sufficient head/media signal-to-noise ratio. Consequently, track densities are required to grow substantially more aggressively than linear densities as the storage industry progresses on the 60% (or higher) areal density growth curve.
As tracks-per-inch (TPI) grow from 25K TPI (10 Gb/in2) to 200K TPI (100 Gb/in2), issues related to data integrity in the presence of operating shock and the ability to servo precisely/quickly on tracks 0.12 xcexcm wide will become substantially more severe. These issues are described, for example, in Chew K.K., xe2x80x9cControl Systems Challenges to High Track Density Magnetic Disk Storagexe2x80x9d, IEEE Trans on Magnetics, Vol. 32, No. 3, May 1996, pp. 1799-1804, and Chew K.K., xe2x80x9cEfficient Adaptive Fast Short Moves for Magnetic Disk Drives Utilizing Magneto-resistive Headsxe2x80x9d, Proceedings of the Second World Congress on Intelligent Control and Intelligent Automation, Jun. 23-27, 1997, pp. 1053-1057.
State-of-art servo technology uses so-called embedded servo position information to derive position information to position the read/write head to the desired read/write position. Position information is embedded within the data track, usually taking up less than 10% of the overall capacity. The servo sampling time ranges (depending on high or low performance) from 100 xcexcs to 200 xcexcs in current technology (10K TPI for high-end and 17K TPI for mobile drives) and is projected to be reduced by square root of increase in TPI.
To position/move the read/write head more accurately/quickly, a higher bandwidth servo system is required. This requires a higher sampling rate and more servo information to be embedded in the data tracks. Since position information is derived from finite discrete servo fields embedded within a data track, the servo control system is essentially operating open-loop in-between servo information. Any shock of sufficient magnitude may force the read/write head to wander significantly off track-center without the controller""s knowledge.
There are two aspects of data integrity in the presence of operating shock. If a shock happens during a read operation, the head will start to read adjacent track information. The error-correcting code (ECC) capability in the drive will indicate invalid information in the data stream, causing the erroneous data sector to be read again. This is a soft-error and affects the performance and throughput in the drive, but does not affect data integrity, as the ECC will detect the error and cause the drive to re-read the erroneously read sector or sectors.
If a shock happens during a write operation with the write-head continuing to write while the head traverses off track-center over adjacent tracks, previous information in adjacent tracks will be written over. This causes a hard-error and inability to correctly retrieve data on current and adjacent tracks. Once data is written onto adjacent tracks, adjacent track data may be altered in a manner which is uncorrectable. Moreover the tracks for which data was intended to be written to may contain old data or an uncorrectable combination old and new data.
Present embedded servo methodology compromises position accuracy (through bandwidth limitations) and data integrity (due to lack of servo information while reading or writing data). Moreover, random access times are typically sacrificed (because of inadequate seeking bandwidth) with higher acoustic noise induced during seeking (due to large step changes in control signals).
In prior art hard drives, when the read/write head is between servo fields, the drive controller does not know whether the read/write head on- or off-track. If an external shock is inflicted on the drive during this time, the read/write head may wander substantially off-track. For example, if the drive sustains a shock of 100 g (typical of opening and closing a desk drawer), it may move more than 100xcexcxe2x80x3 off-track in between servo fields, equivalent to 1-track in a 10K TPI drive. If the head is in a write mode, such an event will lead to catastrophic overwriting and corruption of an adjacent track.
One present solution is to include a shock sensor in the drive that senses external shocks of sufficient magnitude and immediately shuts off writing. See, for example, Levy L., xe2x80x9cSmart Shock Sensors Preserve Data Integrity in Hard Drivesxe2x80x9d, Data Storage Magazine, May/June 1996, pp. 33-39.
The shock sensor approach has at least two disadvantages. First, the shock sensor is itself a mechanical device, having its own mechanical characteristics with regard to shock and vibration. During normal drive seeking operations, the drive may generate internal shocks in excess of 50 g. If a sensor is set to trip above 20 g, the sensor may falsely trigger even in normal drive operations. Moreover, the sensor may take a longer time than the seek settling time for its own vibrations to die away, taking a much longer time than is necessary to remove write inhibit signals. Second, the cost of the shock sensor is an additional cost to the drive. The external shock sensor cost depends on the number of axes required to detect transient shocks.
Another solution is to write more servo information per track, so that off-track position measurements may be made more frequently, enabling the drive to issue a write inhibit command before the read/write head wanders too far off-track to cause catastrophic data corruption during writing. With more servo information per track, a higher servo bandwidth may be achieved, leading to better seeking, settling and tracking performances as well as reduction of acoustic noise during seeking from a higher sampling rate.
Using additional servo information is also an unattractive solution, as it increases the servo overhead and reduces drive format efficiency (i.e., ratio of data sectors to servo sectors). In addition, using additional servo information reduces overall drive capacity and leads to increased cost per megabyte. Consequently, more aggressive areal density design points are required to meet drive capacity requirements, which ultimately adversely affecting final manufacturing yields.
In the prior art, early hard drives were known which provided one or more disk surfaces comprising only servo sectors (a so-called xe2x80x9cdedicatedxe2x80x9d servo surface). Such hard disk drive might comprise a number of disks arranged along a common spindle with a number of read/write heads arranged along a common control arm. One side of one disk might comprise only servo sectors, and no data. That servo disk may be used to track data reads and write to all other surfaces of all other disks in the drive. Such techniques, however, a rarely used today, due to the high overhead associated with using an entire disk surface for servo sectors.
Tung, U.S. Pat. No. 4,924,160, issued May 8, 1980, discloses the use of staggered servo sectors on alternate sides of a disk (FIG. 4). Tung is primarily directed toward a technique for accelerating and decelerating a drive head. Since Tung accelerates the drive head so rapidly, the chances of missing a servo sector while crossing the disk are increased (Col. 2, lines 47-51). Thus, Tung uses a xe2x80x9cstaggered seekxe2x80x9d to reduce the likelihood of missing a servo sector during such a rapid seek.
Tung teaches that all read heads are available during a seek operation as xe2x80x9cthere is no write operation taking place during seek mode in a typical disk drivexe2x80x9d (Col. 3, lines 4-6). Thus, other drive heads may be used for seeking other sectors. Tung teaches switching (xe2x80x9cscanningxe2x80x9d) between read heads during a seek operation (Col. 3, lines 34-41). However, Tung explicitly teaches that his device is not used during short seeks of less than seven tracks. (Col. 3, lines 51-53).
Tung teaches staggered servo sectors, and a seek and settle technique. However, Tung is very narrowly directed toward the idea of using staggered sectors as a seek technique. Tung does not discuss or address shock sensing. Moreover, Tung explicitly teaches away from writing during a read operation (or vice versa).
There remains a need for low-cost and reliable shock-sensing methods in magnetic hard-disk drives to maintain data integrity when a drive is subjected to external shocks. The present invention provides a novel shock-sensing method and apparatus which is both reliable and low-cost, using a the head preamplifier to detect off-track writing.
In the present invention, servo fields for each surface in a disk drive are staggered from one another. Thus, a servo field for one disk surface may coincide with a data field for another surface. By modifying the read/write head amplifier circuitry, a disk drive controller may alternately read servo field information from adjacent disk surfaces, effectively increasing the amount of servo information available to the controller, without increasing the overall amount of servo information per track.